If it is the assumption, at the time you make the bracha rishona, that you are going to keep eating, then that original bracha rishona is still valid (provided you haven't had a hesech ha'daat [halachik distracted attention] in the interim, e.g. by leaving the house - shinui makom). See e.g. https://groups.google.com/forum/#!topic/hilchos-brachos/y4tjLoBP0v8 and http://www.torahdownloads.com/shiur-20417.html (As in the above link based on the Igros Moshe of Rav Moshe Feinstein, zt"L, there is a time limit on a bracha achrona based on how long it takes to digest e.g. 72 minutes - this is not the case with a bracha rishona which could technically be valid all day if there is no hesech hadaas.
(Technically, we're not blessing the food but are addressing G-d and thanking him for it. There is an obligation upon a person before partaking of food or drink to make the appropriate blessing. Upon making the appropriate blessing, the food that one intends to eat no longer requires an additional blessing provided there has not been a hesech hadaas in the interim. The ways there would be a bracha l'vatala (blessing in vain) because of multiple people intending the same food is if one of them blessed without actually eating or blessed again on food that they had already originally intended to potentially eat when they made the original blessing. As long as everyone ate a tiny amount, no one violated a bracha l'vatala even if they originally intended to eat more.)